This invention relates to a sigma-delta analog-to-digital converter for use, for example, with a two wire vortex flowmeter that uses a sensor to detect the vortex frequency occurring according to the flow rate of fluid under measurement, and outputs the measured flow rate to an external device in the form of a current signal that is also used as the operating power source for the external device. More specifically, the invention relates to an improved sigma-delta analog-to-digital converter incorporating means for isolating input signals in order to reduce current consumption.